ER retrieval of Avr9 compromises its elicitor activity consistent with perception of Avr9 at plasma membrane

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The discovery of an endoplasmic reticulum (ER) retrieval motif at the C-terminus of the predicted Cf-9 resistance protein suggested that Cf-9 might function in the ER. To test whether Cf-9 could detect its cognate avirulence protein, Avr9, from the ER, variants of Avr9 with the ER retrieval motifs KDEL or HDEL were transiently expressed in Cf-9 tobacco. Cf-9-mediated necrosis in response to Avr9 tagged at the C-terminus with the KDEL motif was substantially delayed compared with unmodified...[Show more] Avr9. Interestingly, necrosis was completely abolished when Avr9 was tagged with the HDEL motif. Introduction of the S5A mutation, which destroys the sole N-linked glycosylation site of Avr9, ruled out the possibility that an ER-specific pattern of glycosylation prevented a response to the KDEL or HDEL variants of Avr9. These results suggest that Avr9 is perceived at the plasma membrane. They also suggest that HDEL is a more effective ER retrieval motif than KDEL in tobacco.